The invention is based on a method for generating a certain constant pressure in a test liquid of a test apparatus. A method is already known (German Patent Disclosure DE 27 57 318 A1), in which to generate the pressure in the test liquid, a compressed gas is used that is furnished by a pump. As a consequence of the delivery of test liquid to the reservoir, compressed gas is carried out of the reservoir into the atmosphere, which is very expensive when nitrogen is used as the compressed gas. Moreover, the constantly required pumping of the compressed gas by the pump causes pressure fluctuations in the test apparatus, and generates increased noise.